In optical devices which use laser diodes, a high frequency current is sometimes modulated on the laser diode driving current as a countermeasure for optical feedback; but in the prior art an optical signal processing device has been proposed which modulates a high frequency current so as to traverse across the threshold current of the laser diode, causing multiple longitudinal oscillation modes. In this optical signal processing device, by bringing the frequency of the high frequency modulation signals close to the laser resonance frequency (relaxation oscillation frequency), each of the spectral line widths Δλ of the multimode laser light is increased, and coherence is reduced. By means of such an optical signal processing device, noise arising from optical feedback from an optical system can be reduced, without taking into consideration the optical path lengths of the optical system and laser or the relation to the laser oscillation frequency (Patent Reference 1: Japanese Patent Laid-open No. S60-35344).